Annual precipitation amounts vary greatly. The timing and amount of precipitation will change year to year and deviations from the average cannot be reliably predicted. Turfgrass transpirational water demands can exceed rainfall on a seasonal basis, which can lead to water stress, poor plant growth, dormancy, and, in extreme situations, death of established turfgrass. Water stress is amplified in very sandy native soils and sand-based sports fields with inherently low water holding capacities. Therefore, rainfall is typically supplemented with irrigation during the summer and early fall months to maintain adequate soil moisture, particularly during periods of drought.
Although irrigation may be needed, water conservation is also vital for socioeconomic and environmental reasons. As committed environmental stewards, sports field managers regularly conserve water while maintaining healthy, dense turfgrass necessary for the safe uniform surfaces on which athletes play. Balancing these demands with natural resource conservation goals takes experience, training, and a comprehensive understanding of turfgrass management in relation to practical water management, including sound irrigation practices.
Irrigation is used to supplement seasonal water deficiencies to meet plant needs. Ideally, irrigation systems should be designed and managed to accurately and evenly apply just enough water to meet the needs of plants in order to maximize water conservation. Poorly designed and/or managed irrigation systems can lead to under- and over-watering, which can affect plant health, increase pest and disease pressure, waste water, and ultimately lead to unnecessary surface runoff or leaching.
Irrigation may have other uses in addition to meeting the turfgrass system’s physiological needs, such as reducing surface hardness to lower the potential of athlete injury; infield management on baseball/softball fields; watering in fertilizer and pesticides; and cooling the field temperatures, especially on synthetic turf fields.
Irrigation Water Sources
Irrigation water must be dependable, reliable, and of sufficient quantity and quality to accommodate turfgrass grow-in needs and ongoing maintenance. It must also pose no threat to public health. Irrigation water can come from several sources:
Ponds, lakes, or stormwater detention ponds
Groundwater from wells
Any combined supplemental sources from rainwater and stormwater collection
Whenever possible, alternative water sources should be used to conserve freshwater drinking supplies as the routine use of potable water is not a preferred practice. Municipal drinking water should be considered only when no acceptable alternatives exist. Greywater is defined as any water that has been treated after human use and is suitable for limited reuse, including irrigation.
Such water is also referred to as reclaimed, waste, and effluent water. Using greywater may also be part of nutrient reduction strategies to meet total maximum daily load (TMDL) goals in impaired watersheds.
Irrigation Water Quality
Irrigation water should be evaluated to determine its suitability for irrigation and plant growth. Some water sources (especially greywater or water from storage and retention ponds) should be tested regularly to ensure that quality stays within acceptable limits to protect soil quality and turfgrass performance. Water suppliers, especially those providing greywater and municipal water, are often required to do testing to show that water is safe for human contact. These tests often contain information on nutrients, toxins, and salinity. Test results are generally available upon request. For other sources, water samples can be taken by the manager and submitted to a credible laboratory for analysis.
Nutrients and Toxicities
Nutrients and other elements dissolved in irrigation water should be accounted for in nutrient management programs to avoid toxicities and over-fertilization, resulting in plant health problems and/or environmental contamination. Most irrigation sources are derived from water that has passed through or over soil and, thus, has picked up various minerals. Calcium and magnesium carbonates (hard water) are extremely common and found at relatively high concentrations in most irrigation sources. Sulfur, chloride, boron, and, less commonly, nitrogen and potassium can also be high enough to significantly supply plants with these nutrients. Phosphorus is dissolved in some waters to the point of being an environmental concern, but it is very rare for it to be found at levels sufficient to provide for plant needs.
Specific ion toxicities occur when nutrients and other chemical elements are excessively high. For example, chloride and boron are both essential plant nutrients, but they can harm plants if they are present in excessive amounts in irrigation water. When these nutrients are present in significant quantities in irrigation water, the fertilizer rates for each nutrient can be reduced partially or completely. If such nutrients are present in the soil in excess amounts, leaching by irrigating to excess with reasonable quality water may be needed to move these nutrients below the root zone. Careful monitoring and managing these issues may be necessary depending on the timing and amount of local rainfall. Irrigation water quality and soil testing are essential BMPs to help identify these issues.
Salinity and Sodicity
All nutrients and many other chemical elements are “salts.” In addition to needing individual nutrients, plants need salt for proper water regulation. However, salt has a high affinity for water and excess in the soil desiccates plants – even when the soil is saturated with water. This requires salinity management.
Sodium is not an essential nutrient but is considered to be part of the salt complex in soil that can be beneficial to plants. However, soil aggregates containing clays can be destroyed when the salt concentration is high relative to calcium and magnesium. This requires sodicity management.
Some irrigation waters, particularly greywaters, can be saline and/or sodic (Harivandi, 2007). This is relatively more common in arid regions, but groundwater sources in shoreline areas might also be affected due to saltwater intrusion.
Irrigation water quality analyses (as well as routine soil testing) can be used to help identify plant health problems related to salinity and sodicity. Salt tolerant species and/or varieties may need to be used, along with soil remediation management, if irrigation water is saline. Recommendations for correcting salt-affected soils include the following:
Alternative irrigation water sources and/or blending sources to improve quality.
For saline soils, maintain soil moisture at relatively high levels, especially during hot periods.
Provide for drainage. (Salts must have somewhere to go.)
For sodic soils, add a soluble calcium source (e.g., gypsum) before leaching.
Leach the salts below the root zone using reasonable quality water.
Retest the soil and water frequently.
Sports field irrigation systems are designed to be either in-ground or portable systems. In-ground irrigation systems are the most efficient and convenient method and allows for good uniformity when properly designed, installed, and maintained. While the initial expense is typically greater than for portable systems, the increase in efficiency conserves water and reduces labor costs. Portable systems (typically a large water cannon) can be used for rescue during drought or when the main system is off-line during construction/repair. Portable irrigation systems typically require massive flow rates and manual operation and tend to have poor uniformity.
Design and Installation: A properly designed irrigation system requires a professional to design and maximize the distribution uniformity (DU). Architects should require a minimum DU of >75-80%. An irrigation system must have:
Adequate and consistent water pressure.
Properly sized pipes and irrigation zones to provide adequate flow rate.
Efficient sprinkler heads and correct nozzles.
Head-to-head placement of sprinkler heads for 100% overlap.
Incorporating the use of “smart” computerized irrigation controllers can significantly improve water conservation and system performance. The cost and availability of these controllers has become very reasonable. Water savings should more than cover the cost of purchase and installation. These controllers provide many advantages, including remote adjustments of irrigation as needed. The controller should account for:
Weather conditions (evapotranspiration as impacted by temperature, solar intensity, wind speed, precipitation, and humidity).
Soil characteristics (water content and/or potential, water holding capacity).
Irrigation system parameters (application rate unique to each zone).
The controllers should be frequently monitored and adjusted for seasonal changes in turfgrass growth, rooting depth, and variable environmental conditions. Smart controllers typically access evapotranspiration (ET) data as part of their computerized decision-making. Weather stations close to the field better inform smart controllers with more accurate information on evapotranspiration. Soil moisture sensors placed in multiple locations also aid in informing smart controllers.
Irrigation Audits and Maintenance:
Properly working systems are necessary for efficient irrigation. Irrigation efficiency degrades with time as sprinkler head positions are altered, nozzles wear, and leaks develop. Irrigation audits should be conducted regularly by field staff or outside contractors to assess the system function, ensuring that the irrigation system works reliably and cost effectively. Some regions have water organizations, university Extension, or other groups that often sponsor programs to help with these assessments. The Irrigation Association has published irrigation audit guidelines. Irrigation audits measure DU and Scheduling Coefficient, which is the measurement of the average water applied to the driest, most critical parts of an area under test and compares that with the average.
Routine checks of the irrigation system should be conducted to ensure that the system is working as it should be, especially if the DU drops below the optimum. These inspections include looking for broken sprinkler heads, misaligned heads, sunken heads, water pressure irregularities, leaks in the lines or heads, and improper/incomplete rotation. In properly designed, installed, and maintained systems, sprinkler heads specifically designed for use on sports fields are not a hazard to players.
Irrigation decision-making starts with a rudimentary understanding of soil-plant-water relations and then determines how often (frequency) and how much (rate) to apply. An irrigation system should be operated based on the water needs of the turfgrass and not on a calendar-only approach. Water requirements of established turfgrass stands depends on the species, soil type, soil moisture, weather conditions, and time of year.
Understanding Soil-Plant-Water Relations
Water Infiltration: Irrigation systems should not apply water faster than the soil can take it in (infiltration rate). Soil texture impacts water infiltration (estimates shown in Table 4). However, compaction greatly hinders infiltration rates and lowers these values. Cultivation and, when possible, topdressing are critical to enable adequate infiltration rates (and to provide for air exchange). Most soils are subject to compaction, although some have higher potential than others. Sand-based fields that are precisely built and maintained to meet ASTM specifications are resistant to compaction, but even slight increases in clay and silt above these specifications results in a field that is highly prone to compaction. For example, sandy loams are among the soil textures with the highest compaction potential with reduced infiltration rates.
In addition to soil texture, soil organic matter impacts infiltration, improving water and nutrient holding capacity. However, if soil organic matter is elevated in sandy fields, it can greatly decrease water infiltration rates. Proper cultivation and topdressing can help moderate soil organic matter.
Furthermore, soils, especially sands, are prone to developing localized dry spot (LDS) due to hydrophobicity when the soil repels water after extreme drying. In these cases, water infiltration rates are zero, with water running off high spots and accumulating in low areas and/or running off-site. LDS is corrected with treatment with a high-quality wetting agent labeled for turfgrass use.
Water Holding Capacity: Once water infiltrates the soil, it is either held in storage or leaches below the root zone. Water holding capacity is a function of soil texture and organic matter. Clays have a high affinity for water, with silt and sand significantly less. As such, textures with high clay percentages tend to have high water holding capacity. Soil organic matter is also an important component of water holding capacity.
In order to understand irrigation needs, it is important to understand the rule of halves, which is:
About half of an “ideal” soil is comprised of soil pores that hold some combination of water and air.
About half of this water, the gravitational water, is drained after the soil pores are saturated within ~24 hours (less time for sandy soils with little water holding capacity and more time for compacted soils).
About half of this water is plant available, with the other half is held so tightly in the soil that plants are not able to utilize it.
About half of the plant available water can be used (the depletable water) before plants start encountering moisture stress.
Ideally, the soil moisture is replenished with irrigation after the depletable water is utilized. Also, it is important to realize that the ideal soil isn’t common in the urban landscape, especially with compacted soils, but this rule of halves enables a close approximation of irrigation needs.
Evapotranspiration: Stored water is lost through ET, which is the combined transpirational loss of water from plant shoots and evaporative water loss from the soil. The ET increases as solar intensity, temperature, and wind speed increase and relative humidity decreases. In general, water lost to ET should be replaced through precipitation and/or irrigation, although plants are able to survive to varying levels if water deficits exist temporarily.
In general, it is common to have maximum ET rates of about 0.25” to 0.33” of water loss per day. ET losses during cool times of the year can approach zero. Thus, it is vital to understand these losses in order to efficiently irrigate. ET rates can be gathered from various government, university, and private websites or with onsite weather stations.
Water Needs Vary by Species/Variety: There are differences between turfgrass species, and varieties within species, with regard to the amount of water needed to achieve optimal health and function. In general, the largest difference exists between cool and warm season species, with the latter tending to require relatively less total water. Although there is variability due to a host of situations and circumstances, within cool season species, the general order of water requirements from least to greatest is as follows: fine fescue, perennial ryegrass, turf-type tall fescue, Kentucky bluegrass, and annual ryegrass. Within warm season species, the general order of water requirements from least to greatest is as follows: buffalograss, zoysiagrass, bermudagrass, seashore paspalum, and bahiagrass.
Cultural management practices or environmental factors that result in a significant change in leaf area or shoot density of a given species may have a significant impact on the relative rankings compared to other species.
For more information on selecting the best turfgrass species/variety for site specific conditions, including irrigation needs, see the Turfgrass Establishment chapter. Selection should be based on water efficient/drought resistant evaluations provided by the National Turfgrass Evaluation Program, Turfgrass Conservation Alliance, and the Alliance for Low-Input Sustainable Turf.
Root Depth: The effective root zone is defined as the depth to which a large majority of the root system exists. This changes throughout the season and with maturity. Plant health status also impacts root depth, with poor health potentially resulting in a poor root system. For example, root-attacking insects and nematodes and deficiencies/excesses in nutrients (especially N and P) affect root depth.
Kentucky bluegrass is a common species in cool season and transition zone areas. Much like other cool season grasses, it is relatively shallow rooted and water inefficient. In contrast, hybrid bermudagrass is the most common sports turfgrass species used in warm season areas. It is relatively water efficient as it has deep and prolific rooting and relatively low water use rates. Its effective root zone is approximately double that of Kentucky bluegrass, which allows for more efficient water use.
A soil’s water holding capacity within the effective root zone is used to estimate the soil water holding reservoir available to the plant. Irrigation water should be applied to the depth of the root zone or slightly below. Water that passes below can wick back upward, but water is wasted if leaching is excessive.
In general, turfgrass should be irrigated as infrequently as possible without causing drought stress injury. This produces higher quality turfgrass while saving water and money.
Approximately half of the total water holding capacity of the soil is “plant available.” Approximately half of the plant available water can be utilized before plants begin experiencing moisture stress. For example, a silty clay loam soil with 2% organic matter is determined to have 4” of total water holding capacity in one foot of soil. According to the rule of halves, about half of this is plant available. Of this 2” of plant available water, about half (1”) of this is the depletable water that can be utilized before irrigation is needed. However, short-mowed turfgrass roots in sports field root zones rarely exist down to one foot. In this case if, for example, the effective root zone is 6” then only 0.5” can be lost to ET before irrigation is needed in order to avoid stress. In contrast, a loamy sandy soil with very little organic matter could easily have half of this amount.
Some older systems utilize an irrigation clock that runs daily or multiple times a day. This calendar-based approach is not good for grass health, surface playability, or environmental stewardship. Rather, soil type, root depth, and weather should determine irrigation frequency. Depending on weather, sports field managers may irrigate every 10 to 20 days in the spring and every two to five days in the summer without any negative impacts.
Three basic approaches are used to determining irrigation frequency. The first is visual, with daily or multiple times daily in-person evaluations. This is done by watching the grass and irrigating once the grass appears to be stressed (dull greyish-bluish color and doesn’t bounce back when stepped upon). This approach can be a problem if the grass slides into stress immediately prior to an event. This visual approach is relatively more common in areas where irrigation is not as commonly needed and/or for venues with relatively low profile. Collegiate and professional venues and areas where irrigation is routinely needed generally require a more sophisticated approach.
The second approach is commonly referred to as the “checkbook” or ET method. This refers to the water holding capacity of the effective root zone, which shifts as a function of many variables that change over the course of a growing season. A smart irrigation controller greatly assists in using this method. An automated system can account for soil properties and weather, but typically do not account for rooting depth changes during the season. Therefore, monthly assessments should be used to adjust the system. Smart controllers’ accuracy is improved with data from onsite weather stations.
The third approach is a more sophisticated and potentially more accurate method that involves measuring moisture in the root zone with sensors. This is critical to understanding turfgrass water needs. Sensor technology is changing rapidly with newer models providing greater accuracy and more options. There are two approaches to measuring soil moisture:
Water content sensors indicate the total volumetric water content in the soil including plant available water or, in other words, the moisture content at the point the plant wilts. The percent moisture at which irrigation is triggered is variable with soil properties.
Soil water potential (or tension) sensors are a recent development to measure the energy of water. These are an improvement over water content sensors used alone in that their output and interpretation is not dependent upon soil properties and, thus, is more accurate and easier to use to trigger irrigation events.
Irrigation Amount and Rate The amount of irrigation water to be applied needs to bring the bottom of the root zone to field capacity. If the root zone becomes saturated, water will continue to move downward as gravitational water and will potentially be lost for plant use. Generally, the amount that is needed is the depletable water. Using the previous examples, the silty clay loam would need 0.5” and the sandy loam would need 0.25” to be applied as irrigation to replace the depletable water in the effective root zone.
In addition to consideration of the total amount of water to apply, water should not be applied faster than the soil can take it in (infiltration rate). Infiltration rate tables are available, although the rate is so variable that it is a good practice to observe when water ceases to infiltrate and begin to run off. If this occurs before the total amount of water needed is applied, then adjustments need to be made. Cultivation will improve infiltration rate. In severe cases, low flow heads/nozzles should be used. Alternatively, irrigation can be cycled intermittently by shutting it off temporarily to allow for some infiltration and drainage before applying the remainder of the total amount needed to move water to the bottom of the root zone. This cycling can be repeated as many times as necessary in order to apply the desired total rate. Multiple cycle irrigation controllers can be programmed to do this automatically in installed systems.
Early morning is typically considered the best time to irrigate, when wind speed and evaporation rates are low. Watering late in the evening or at night extends the time that leaves remain wet, significantly increasing the chance for disease. Midafternoon watering can lead to nonuniform distribution and evaporative losses if wind speeds and/or temperatures are high.
In addition to these considerations, the sports field manager must also contend with the usage schedule to determine the best time to irrigate. Fields should not be too wet during play to minimize slippage and compaction.
Irrigation Best Management Practices
Irrigation Water Supply Best Management Practices
When possible, use alternative water supplies/sources that are appropriate and sufficiently available to supplement water needs.
Water mains must have a thorough cross-connection and backflow prevention device in place that operates correctly.
Irrigation Water Quality Best Management Practices
Assess the irrigation water quality (salinity, sodicity, toxicities, and nutrient content) by obtaining lab data from the water supplier or taking and submitting samples to a reliable laboratory.
Account for the nutrients in irrigation water when making fertilizer calculations.
If the water source is saline, use salt-tolerant turfgrass varieties to mitigate saline conditions and/or leach salts from the soil.
If water is sodic, add a soluble calcium source and leach.
Irrigation Systems Best Management Practices
Maximize the DU in the design and maintenance of the irrigation system. Architects should require a minimum DU (>75-80%).
Incorporate the use of “smart” computerized irrigation controllers to significantly improve water conservation and system performance.
Frequently monitor controllers and adjust for seasonal changes in turfgrass growth, rooting depth, and variable environmental conditions.
Routinely check irrigation system (broken sprinkler heads, misaligned heads, sunken heads, water pressure irregularities, leaks in the lines or heads, and improper/incomplete rotation) to ensure that the system is working as designed, especially if the DU drops below the optimum.
Regularly conduct pre-season audits to assess the system function, ensuring that the irrigation system works reliably and cost effectively.
Irrigation Decision-Making Best Management Practices
Intentionally allow grass to be mildly moisture stressed twice during spring to promote deeper rooting.
Consider water use and drought resistance when selecting turfgrass species and varieties.
Avoid keeping the soil saturated to promote deeper rooting.
Evaluate ET losses daily and irrigate if needed.
In conjunction with ET, evaluate soil moisture using soil moisture sensors, preferably with water potential sensors used alone or in combination with water content sensors.
Check rooting depth throughout the season and irrigate to the depth of rooting (i.e., water deeply and infrequently).
If events allow, do not irrigate until the depletable water is utilized—allowing soil to dry to minimize disease and root oxygen deficiency.
Apply enough water so that the bottom of the root zone reaches field capacity without reaching saturation in order to minimize leaching losses.
Do not apply water faster than soil infiltration. Use cultivation, low flow nozzles, and/or intermittent cycling to minimize water runoff.
Water during morning hours when feasible and when the field schedule allows.